The pyrido[2,1-a]isoquinoline derivatives of the formula
whereinR2, R3 and R4 are each independently selected from the group consisting of hydrogen, halogen, hydroxy, lower alkyl, lower alkoxy and lower alkenyl, wherein lower alkyl, lower alkoxy and lower alkenyl may optionally be substituted by a group consisting of lower alkoxycarbonyl, aryl and heterocyclyl, and the pharmaceutically acceptable salts thereof are disclosed in PCT International Patent Appl. WO 2005/000848.
A major task in the synthesis of the compounds of formula II is the introduction of the chiral (S)-4-fluoromethyl-pyrrolidino residue which in the current synthesis according to the PCT Int. Appl. WO 2005/000848 involves coupling of a suitably protected tricyclic amine moiety with a racemic side chain building block (i.e with rac-4-chloro-3-fluoromethylbutyryl chloride) and isolation of the desired isomer from the ca. 1:1 isomer mixture by chromatographical separation. Such a chromatographical step is difficult to carry out on large technical scale and furthermore a yield of maximally ca. 50% can be achieved only. The problem to be solved therefore was to find a suitable process alternative which affords a higher yield and which can be conducted on technical scale.